An HI line search for optically identified dwarf galaxy candidates in the M81 group

Sensitive 21cm H I line observations were performed for 23 dwarf members and possible members of the nearby M 81 group of galaxies, including five objects of a clustering of extremely low-surface brightness objects of unknown nature. With the Nancay decimetric radio telescope the radial velocity range of -529 to 1826 km s(-1) was searched to an rms noise of similar to 2 - 4 mJy. Only three objects were detected. However their high radial velocities (between 600 and 1150 kms(-1)) show them to lie behind the M 81 group. These three objects, classified a;s dSO: (UGC 4998) and Im (Kar 1N and UGC 5658), have HI masses of 0.4, 1.6 and 2.0 10(8) M.; for the assumed distance of 4 Mpc, and HI mass-to-blue light ratios of 0.04, 0.73 and 0.18 M./L.,(B); respectively. Considering that half of the observed objects are classified as irregular dwarfs, hence expected to be relatively gas-rich, the resulting detection rate of about 1/3 is quite low. However, the mean redshift and velocity dispersion of the M 81 group V = 101 km s(-1), sigma = 114 km s(-1)) suggest that the H I emission of low velocity H I - rich members of the M 81 group may still remain hidden within the strong Galactic HI emission (typically -150 less than or similar to V less than or similar to 115 kms(-1)) or; for the 6 dwarf candidates in the immediate vicinity of M 81, overshadowed by the very extended HI envelope encompassing M 81; M 82, NGC 3077, and NGC 2976 (-280 less than or similar to V less than or similar to 355 km s(-1)).